..\OBJ\PWM.axf: Error: L6218E: Undefined symbol EXTI_ClearITPendingBit (referred from exti.o).

This error message is indicating that the linker was unable to find a definition for the symbol "EXTI_ClearITPendingBit", which is being referred to in the "exti.o" object file. This symbol is most likely a function or variable that is defined in another source file, but has not been included in the build process.

To resolve this issue, you will need to ensure that the source file containing the definition for "EXTI_ClearITPendingBit" is included in your build process. This may require adding additional source files to your project or modifying your build settings to include the necessary files. Alternatively, you may need to check that the function or variable is properly declared and defined in the source file where it is used.